Heel When Turning.—When the helm is first put over, the pressure on the rudder causes a small inward heel. As the rota tional speed of the ship increases the small inward heel is suc ceeded by a steady outward heel, caused by the couple formed by the centrifugal force and the lateral resistance diminished by the (usually) small couple due to the rudder pressure. If when turning, the helm be put quickly amidships, the opposing couple due to rudder pressure is re moved, and the outward heel is momentarily increased. Instances have occurred of ships with small stability and comparatively large "rudder couple" capsizing through this cause.
Types of Rudders.—Rudders used in ships are of two general types (I) unbalanced, and (2) balanced.
The unbalanced rudder, supported at its forward edge, is in stable equilibrium when amidships ; and for this reason and its simplicity of construction is pre ferred when the force required to put the rudder hard over is sufficiently moderate to enable steering to be performed by hand or by an engine and gear of moderate size. For high speeds and large manoeuvring powers the unbalanced rudder is gener ally unsuitable; and balanced rudders, in which about one quar ter of the area is usually placed before the axis, are adopted in order to reduce the force required and the work done to obtain large angles of helm. A balanced rudder is unstable amidships, and if left free, comes to rest at a moderate angle on either side of the middle line. Unbalanced rudders, extending up to, or above the waterline and compara tively narrow longitudinally, are generally fitted in the merchant service (see figure 4). Somewhat greater efficiency when using small or moderate angles of helm is obtained with rudders of this type, as for a given pressure on the rudder, the turning moment on the rudder and consequently the power required in the steering gear is less. For fast liners a type of balanced rudder is sometimes adopted (see figure 5).
For warships where the steering gear has to be kept below waterline for protection, broader and shallower rudders are adopted; and for the reason above stated balanced rudders are now practically universal in warships (see figure 6).
When a ship is going astern manoeuvring is performed with some uncertainty and additional rudders have sometimes been fitted at the bow ; these are generally for use in confined waters where it may be necessary to go astern for a considerable period. Rudders of special form and characteristics such as the Kitchen rudder and the Flettner rudder, have been fitted to some modern vessels.
Experimental Results.—Experiments have been made to ascertain the effects of angle of helm, time of putting helm over, and draught and trim of the ship, on the turning properties of the vessel. In general it is found that the tactical diameter di minishes with increase of helm angle. In ships having unbal anced rudders and hand steering gear considerable time is required to put the helm over at full speed ; consequently the tactical diameter and the advance are greater at high speeds than at low speeds. With power worked steering gear with which the helm can be put hard over in from 10-20 seconds at any speed, the speed is found to have little influence on the path described when turning. In the case of torpedo boat destroyers a marked increase in tactical diameter and in advance occurs at high speeds. A moderate variation in mean draught has little effect, but additional trim by the stern results in a greater space being required for turning.
By working one propeller ahead and the other astern the space required for turning may be shortened, but the time of turning is frequently increased. The path described is generally tortuous in character and depends on the relation between the revolutions of the propellers. In a single-screw ship, with the propeller well immersed, the upper blades experience greater resistance to rota tion than the lower blades ; hence a right-handed screw tends to turn the ship's head to starboard, and starboard helm is necessary. The reverse is occasionally experienced when the upper portion of the screw is incompletely immersed.